Structurally viscous powder clearcoat slurry free from organic solvents and external emulsifiers, method for producing the same the use thereof

ABSTRACT

A pseudoplastic powder clearcoat slurry free from organic solvents and external emulsifiers and comprising particles which are solid and/or of high viscosity and are dimensionally stable under storage and application conditions, where the particles have an average size of from 1.0 to 20 μm, at least 99% of the particles having a size ≦30 μm, and comprise as binder at least one polyol with an OH number &gt;110 mg KOH/g, containing potentially ionic groups, and where the powder clearcoat slurry has a potentially ionic group content of from 0.05 to 1 meq/g of solids, at a degree of neutralization of not more than 50% contains from 0.005 to 0.1 meq/g of solids of ionic groups produced by neutralization of the potentially ionic groups, and has a viscosity of (i) from 50 to 1 000 mPas at a shear rate of 1 000 s −1 , (ii) from 150 to 8 000 mPas at a shear rate of 10 s −1 , and (iii) from 180 to 12 000 mPas at a shear rate of 1 s −1 .

CROSS REFERENCE TO RELATED APPLICATIONS

This applications is a National Phase Applications of Patent ApplicationPCT/EP01/12925 filed on 8 Nov. 20001, which claims priority on DE 100 55464.4, filed on 9 Nov. 2000.

The present invention relates to a novel powder clearcoat slurry freefrom organic solvents and external emulsifiers which possessespseudoplasticity. The invention relates not least to the use of thenovel powder clearcoat slurry for automotive OEM finishing andrefinishing, the interior and exterior coating of constructions, thecoating of doors, windows and furniture, and industrial coating,including coil coating, container coating and the impregnation and/orcoating of electrical components.

BACKGROUND

The German patent DE 198 41 842 C2 discloses a powder clearcoat slurrywhich is free from organic solvents and external emulsifiers andcomprises solid spherical particles with an average size of from 0.8 to20 μm and a maximum size of 30 μm, said slurry having an ion-forminggroup content of from 0.05 to 1 meq/g, a neutralizing agent content offrom 0.05 to 1 meq/g, and a viscosity of

-   (i) from 50 to 1 000 mPas at a shear rate of 1 000 s⁻¹,-   (ii) from 150 to 8 000 mPas at a shear rate of 10 s⁻¹,-   (iii) from 180 to 12 000 mPas at a shear rate of 1 s⁻¹.    The binder used is a methacrylate copolymer having an OH number of    110 mg KOH/g (cf. Preparation Example 1 on page 6 lines 30 to 47 of    the patent). The powder clearcoat slurry of Example 1 of the patent    (cf. page 7 lines 31 to 54 of the patent), prepared using this    binder, has a carboxyl group content of 0.52 meq/g of solids and a    carboxylate group content of 0.22 meq/g of solids, from which the    degree of neutralization is calculated as 42%. The particle size is    6 μm. The powder clearcoat slurry of Example 2 of the patent (cf.    page 7 line 56 to page 8 line 22 of the patent), prepared using the    binder, has a carboxyl group content of 0.19 meq/g of solids and a    carboxylate group content of 0.10 meq/g of solids, giving a degree    of neutralization of 52%.

The known powder clearcoat slurry is prepared by

-   1) emulsifying an organic solution comprising binder and crosslinker    to give an emulsion of the oil-in-water type,-   2) removing the organic solvent or the organic solvents, and-   3) replacing by water some or all of the volume of solvent removed,    to give a powder clearcoat slurry comprising solid spherical    particles,    where-   4) additionally, at least one ionic, especially anionic, thickener    and at least one nonionic associative thickener are added to the    powder clearcoat slurry.

This powder clearcoat slurry can be prepared with a small number ofprocessing steps; on the basis of its typical powder slurry properties,with residual solvent contents of <1%, and its particle sizes, itexhibits advantageous application characteristics. Even without theassistance of organic solvents, there is generally no popping at therequired film thicknesses of approximately 40–50 μm. Moreover, theparticles of the slurry, owing to mixing of their constituents insolution, are very homogeneous.

However, the clearcoats produced from the slurry do exhibit blushingwhen exposed to moisture. Furthermore, they do not achieve the requiredchemical resistance of clearcoats produced from customary and known,commercial two-component clearcoat materials.

Attempts to eliminate these disadvantages by raising the crosslinkingdensity of the clearcoats are accompanied by new problems. Thecorresponding powder clearcoat slurries no longer dry in powder form,and at relatively high coat thicknesses, after curing, show film defectsin the form of popping marks and stress cracks.

DESCRIPTION

It is an object of the present invention to provide a new powderclearcoat slurry from which the disadvantages of the prior art are nowabsent and which instead, while continuing to have all of the advantagesof the known powder clearcoat slurry, dries in powder form afterapplication, even at high coat thicknesses, and gives clearcoats whichdo not blush on exposure to moisture, do not display film defects suchas popping marks and stress cracks (mudcracking) above a dry filmthickness of 50 μm, even without the assistance of organic solvents, andhave a chemical stability like that of clearcoats produced from thecommercial two-component clearcoat materials.

Accordingly we have found the novel pseudoplastic powder clearcoatslurry free from organic solvents and external emulsifiers andcomprising particles which are solid and/or of high viscosity and aredimensionally stable under storage and application conditions, where

-   1. the particles    -   1.1 have an average size of from 1.0 to 20 μm, at least 99% of        the particles having a size ≦30 μm, and    -   1.2 comprise as binder at least one polyol with an OH        number >110 mg KOH/g, containing potentially ionic groups,        and where-   2. the powder clearcoat slurry    -   2.1 has a potentially ionic group content of from 0.01 to 1        meq/g of solids,    -   2.2 at a degree of neutralization of not more than 50% contains        from 0.005 to 0.1 meq/g of solids of ionic groups produced by        neutralization of the potentially ionic groups, and    -   2.3 has a viscosity of (i) from 50 to 1 000 mPas at a shear rate        of 1 000 s⁻¹, (ii) from 150 to 8 000 mPas at a shear rate of 10        s⁻¹, and (iii) from 180 to 12 000 mPas at a shear rate of 1 s⁻¹.

In the text below, the novel, pseudoplastic powder clearcoat slurry freefrom organic solvents and external emulsifiers is referred to for shortas the “slurry of the invention”.

In the light of the prior art, it was surprising and unforeseeable bythe skilled worker that the object on which the present invention isbased might be achieved firstly by replacing the binder of the knownpowder clearcoat slurry by a binder which has a higher OH number andsecondly by the possibility of overcompensating for the resultantdisadvantages by a reduction in the level of ionic groups in thebinders, giving overall a profile of properties which exceeded that ofthe known powder clearcoat slurry.

For the slurry of the invention it is essential that the average size ofthe solid particles is from 1 to 20 μm and, with particular preference,from 3 to 15 μm. By the average particle size is meant the 50% medianvalue as determined by the laser diffraction method, i.e., 50% of theparticles have a diameter ≦the median value and 50% of the particleshave a diameter ≧the median value. At least 99% of the particles have asize <30 μm.

Slurries having average particle sizes of this kind and a solventcontent of <1% exhibit better application properties and, at the appliedfilm thicknesses of >30 μm as presently practiced in the automotiveindustry for the finishing of automobiles, exhibit much less of atendency toward popping and mudcracking than conventional powderclearcoat slurries.

The upper limit on particle size is reached when the size of theparticles means that they are no longer able to flow out fully onbaking, and thus the film leveling is adversely affected. In cases whererequirements regarding the appearance are not very stringent, however,the limit may also be higher. 30 μm is considered a sensible upperlimit, since above this particle size the spray nozzles of the highlysensitive application apparatus may become blocked.

The slurry of the invention is free from organic solvents. In thecontext of the present invention this means that it has a residualvolatile solvent content of <1% by weight, preferably <0.5% by weight,and with particular preference <0.2% by weight. In accordance with theinvention it is of very particular advantage if the residual content isbelow the gas chromatography detection limit.

In the context of the present invention, the expression “free fromexternal emulsifiers” is to be understood in the same way.

The above-described particle sizes for use in accordance with theinvention are obtained even without the aid of additional externalemulsifiers if the slurry of the invention contains from 0.05 to 1,preferably from 0.05 to 0.9, more preferably from 0.05 to 0.8, withparticular preference from 0.05 to 0.7, and in particular from 0.05 to0.6 meq/g of solids of potentially ionic groups.

At a degree of neutralization of not more than 50%, preferably not morethan 48%, the slurry of the invention contains from 0.005 to 0.1,preferably from 0.005 to 0.099, more preferably from 0.005 to 0.098, andin particular from 0.005 to 0.097 meq/g of ionic groups produced byneutralization of the potentially ionic groups.

In general, therefore, the chemical nature of the binder is notrestrictive provided it comprises ion-forming groups which areconvertible by neutralization into salt groups and so are able to takeon the function of ionically stabilizing the particles in water.

Suitable anion-forming groups are acid groups such as carboxylic,sulfonic or phosphonic acid groups. Accordingly, the neutralizing agentsused are bases, such as alkali metal hydroxides, ammonia, or amines.Alkali metal hydroxides are suitable for use only to a limited extent,since the alkali metal ions are nonvolatile on baking and, owing totheir incompatibility with organic substances, may cloud the film andlead to instances of loss of gloss. Consequently, ammonia or amines arepreferred. In the case of amines, preference is given to tertiaryamines. By way of example, mention may be made ofN,N-dimethylethanolamine or aminomethylpropanolamine (AMP).

Suitable cation-forming groups are primary, secondary or tertiaryamines. Accordingly, neutralizing agents used are, in particular, lowmolecular mass organic acids such as formic acid, acetic acid,dimethylolpropionic acid or lactic acid.

Binders which contain cation-forming groups are known from the field ofelectrodeposition coating materials. By way of example, reference may bemade to the patents EP 0 012 463 A1, EP 0 612 818 A1 or U.S. Pat. No.4,071,428 A.

For the preferred use of the slurry of the invention as unpigmentedclearcoat materials in automotive finishing, preference is given topolymers or oligomers containing acid groups as ion-forming groups,since these so-called anionic binders are generally more resistant toyellowing than the class of the cationic binders.

Nevertheless, cationic binders with groups convertible into cations,such as amino groups, are likewise suitable for use in principleprovided the field of use is tolerant of their typical secondaryproperties, such as their tendency toward yellowing.

As binders which contain anion-forming groups, it is possible to use anydesired resins containing the abovementioned acid groups. However, it isessential that they also carry hydroxyl groups; i.e., that they arepolyols.

In accordance with the invention, the polyols have an OH number >110,preferably from 120 to 180, and in particular from 130 to 160 mg KOH/g.

Examples of suitable binders are hydroxyl-containing, random,alternating and/or block, linear and/or branched and/or comb addition(co)polymers of ethylenically unsaturated monomers, or polyadditionresins and/or polycondensation resins. For further details of theseterms, reference is made to Römpp Lexikon Lacke und Druckfarben, GeorgThieme Verlag, Stuttgart, N.Y., 1998, page 457, “Polyaddition” and“Polyaddition resins (polyadducts)”, and also pages 463 and 464,“Polycondensates”, “Polycondensation” and “Polycondensation resins”, andpages 73 and 74, “Binders”.

Examples of suitable addition (co)polymers are (meth)acrylate(co)polymers or partially saponified polyvinyl esters, especially(meth)acrylate copolymers.

Examples of suitable polyaddition resins and/or polycondensation resinsare polyesters, alkyds, polyurethanes, polylactones, polycarbonates,polyethers, epoxy resins, epoxy resin-amine adducts, polyureas,polyamides, polyimides, polyester-polyurethanes, polyether-polyurethanesor polyester-polyether-polyurethanes.

In addition to the hydroxyl groups, the oligomers and polymers may alsocontain other functional groups such as acryloyl, ether, amide, imide,thio, carbonate, or epoxide groups, provided they do not disrupt thecrosslinking reactions.

These oligomers and polymers are known to the skilled worker, and manysuitable compounds are available on the market.

In accordance with the invention, the (meth)acrylate copolymers, thepolyesters, the alkyd resins, the polyurethanes and/or the acrylatedpolyurethanes are of advantage and are therefore used with preference.

Highly suitable (meth)acrylate copolymers and processes for preparingthem are described, for example, in the European patent application EP 0767 185 A1, in the German patents DE 22 14 650 B1 or DE 27 49 576 B1 andin the American patents U.S. Pat. No. 4,091,048 A, U.S. Pat. No.3,781,379 A, U.S. Pat. No. 5,480,493 A, U.S. Pat. No. 5,475,073 A orU.S. Pat. No. 5,534,598 A or in the standard work Houben-Weyl, Methodender organischen Chemie, 4th Edition, volume 14/1, pages 24 to 255, 1961.Suitable reactors for the copolymerization are the customary and knownstirred vessels, cascades of stirred vessels, tube reactors, loopreactors or Taylor reactors, as described, for example, in the patentapplications DE 1 071 241 B1, EP 0 498 583 A1 or DE 198 28 742 A1 or inthe article by K. Kataoka in Chemical Engineering Science, Volume 50,Number 9, 1995, pages 1409 to 1416.

Highly suitable polyesters and alkyd resins and their preparation aredescribed, for example, in the standard work Ullmanns Enzyklopädie dertechnischen Chemie, 3rd Edition, volume 14, Urban & Schwarzenberg,Munich, Berlin, 1963, pages 80 to 89 and pages 99 to 105, and in thefollowing books: “Résines Alkydes-Polyesters” by J. Bourry, Paris,Dunod, 1952, “Alkyd Resins” by C. R. Martens, Reinhold PublishingCorporation, New York, 1961, and “Alkyd Resin Technology” by T. C.Patton, Interscience Publishers, 1962.

Highly suitable polyurethanes and/or acrylated polyurethanes and theirpreparation are described, for example, in the patents EP 0 708 788 A1,DE 44 01 544 A1, or DE 195 34 361 A1.

The binders described above may be used individually or as a mixture ofat least two different binders. In accordance with the invention, the(meth)acrylate copolymers afford particular advantages and are thereforeused with particular preference.

The amount of the above-described binders in the slurry of the inventionmay vary widely. The amount is preferably from 5 to 80, more preferablyfrom 6 to 75, with particular preference from 7 to 70, with veryparticular preference from 8 to 65, and in particular from 9 to 60% byweight, based in each case on the solids of the powder slurry of theinvention.

Suitable crosslinking agents are all crosslinking agents that arecustomary in the field of light-stable clearcoats. Examples of suitablecrosslinking agents are

-   amino resins, as described for example in Römpp Lexikon Lacke und    Druckfarben, Georg Thieme Verlag, 1998, page 29, “Amino resins”, in    the textbook “Lackadditive” [Additives for coatings] by Johan    Bieleman, Wiley-VCH, Weinheim, N.Y., 1998, pages 242 ff., in the    book “Paints, Coatings and Solvents”, second, completely revised    edition, edited by D. Stoye and W. Freitag, Wiley-VCH, Weinheim,    N.Y., 1998, pages 80 ff., in the patents U.S. Pat. No. 4,710,542 A1    or EP 0 245 700 A1, and in the article by B. Singh and coworkers,    “Carbamylmethylated Melamines, Novel Crosslinkers for the Coatings    Industry”, in Advanced Organic Coatings Science and Technology    Series, 1991, volume 13, pages 193 to 207;-   carboxyl-containing compounds or resins, as described for example in    the patent DE 196 52 813 A1 or 198 41 408 A1, especially    1,12-dodecanedicarboxylic acid,-   resins or compounds containing epoxide groups, as described for    example in the patents EP 0 299 420 A1, DE 22 14 650 B1, DE 27 49    576 B1, U.S. Pat. No. 4,091,048 A or U.S. Pat. No. 3,781,379 A;-   tris(alkoxycarbonylamino)triazines, as described in the patents U.S.    Pat. No. 4,939,213 A, U.S. Pat. No. 5,084,581 A, U.S. Pat. No.    5,288,865 A or in the patent application EP 0 604 922 A;-   blocked polyisocyanates, as described for example in the patents    U.S. Pat. No. 4,444,954 A1, DE 196 17 086 A1, DE 196 31 269 A1, EP 0    004 571 A1 or EP 0 582 051 A1; or-   beta-hydroxyalkylamides such as N,N,N′,N′-tetrakis    (2-hydroxyethyl)adipamide or N,N,N′,N′-tetrakis    (2-hydroxypropyl)adipamide.

The crosslinking agents described above may be used individually or as amixture of at least two crosslinking agents. In accordance with theinvention, the blocked polyisocyanates and/ortris(alkoxycarbonyl-amino)triazines afford particular advantages and aretherefore used with particular preference.

The amount of crosslinking agent in the slurry of the invention maylikewise vary widely and is guided primarily by the functionality andamount of the binders on the one hand and by the functionality of thecrosslinking agents on the other. The amount is preferably from 20 to95, more preferably from 25 to 94, with particular preference from 30 to93, with very particular preference from 35 to 92, and in particularfrom 40 to 90% by weight, based in each case on the solids of the slurryof the invention.

The slurry of the invention comprises nonionic and ionic thickeners.This effectively counters the tendency of the comparatively large solidparticles toward sedimentation.

Examples of nonionic thickeners are hydroxyethylcellulose and polyvinylalcohols. Nonionic associative thickeners are likewise available on themarket in diverse selection. They generally consist of water-dilutablepolyurethanes, which are the reaction products of water-solublepolyetherdiols, aliphatic diisocyanates and monofunctional hydroxycompounds containing an organophilic radical.

Likewise commercially available are ionic thickeners. These usuallycontain anionic groups and are based in particular on specialpolyacrylate resins containing acid groups, some or all of which mayhave been neutralized.

Examples of suitable thickeners for use in accordance with the inventionare known from the text book “Lackadditive” by Johan Bieleman,Wiley-VCH, Weinheim, N.Y., 1998, pages 31 to 65.

For the slurry of the invention it is advantageous if both of theabove-described types of thickener are present therein. The amount ofthickeners to be added and the ratio of ionic to nonionic thickener isguided by the desired viscosity of the slurry of the invention, which inturn is determined by the required sedimentation stability and by thespecial requirements of spray application. The skilled worker willtherefore be able to determine the amount of the thickeners and theratio of the thickener types to one another on the basis of simpleconsiderations, possibly with the aid of preliminary tests.

According to the invention, a viscosity range of from 50 to 1 500 mpasat a shear rate of 1 000 s⁻¹ and from 150 to 8 000 mPas at a shear rateof 10 s⁻¹, and also from 180 to 12 000 mPas at a shear rate of 1 s⁻¹ isset.

This viscosity behavior, known as “pseudoplasticity”, describes a statewhich does justice both to the requirements of spray application, on theone hand, and to the requirements in terms of storage and sedimentationstability, on the other: in the state of motion, such as when pumpingthe slurry of the invention in circulation in the ring circuit of thecoating installation and when spraying, for example, the slurry of theinvention adopts a state of low viscosity which ensures easyprocessability. Without shear stress, on the other hand, the viscosityrises and thus ensures that the coating material already present on thesubstrate to be coated has a reduced tendency to form runs on verticalsurfaces. In the same way, a result of the higher viscosity in thestationary state, such as during storage, for instance, is thatsedimentation of the solid particles is largely prevented or that anyslight degree of settling of the powder slurry of the invention duringthe storage period can be removed again by agitation.

In addition to the essential constituents described above, the solidparticles of the slurry of the invention may comprise additives ascommonly used in clearcoat materials. In this context it is essentialthat these additives do not substantially lower the glass transitiontemperature Tg of the binders.

Examples of suitable additives are polymers, crosslinking catalysts,defoamers, adhesion promoters, additives for improving substratewetting, additives for improving surface smoothness, flatting agents,light stabilizers, corrosion inhibitors, biocides, flame retardants, andpolymerization inhibitors, especially photoinhibitors, as described inthe book “Lackadditive” by Johan Bielemann, Wiley-VCH, Weinheim, N.Y.,1998.

Crosslinking components of polyol type, reactive diluents or levelingassistants which may be incorporated by crosslinking in the film may beadded to the slurry of the invention. It is important, however, thatthese components are located preferably in the external, aqueous phaseof the slurry of the invention and not in the disperse organic phase,where they would bring about a lowering of the glass transitiontemperature Tg and thus coalescence or coagulation of any sedimentedparticles. Examples of suitable constituents of this kind are thermallycurable reactive diluents such as positionally isomericdiethyloctanediols or hydroxyl-containing hyperbranched compounds ordendrimers as described in the German patent applications DE 198 05 421A1, DE 198 09 643 A1, and DE 198 40 405 A1.

Moreover, the slurry of the invention may also comprise constituentscurable with actinic radiation, if it is to be curable both thermallyand with actinic radiation. By actinic radiation is meantelectro-magnetic radiation such as near infrared, visible light, UVradiation or X-rays, especially UV radiation, or corpuscular radiationsuch as electron beams. The conjoint use of heat and actinic radiationis also known as dual cure.

In order to obtain a dual-cure slurry of the invention, it is possibleto incorporate (meth)acrylate, ethacrylate, crotonate, cinnamate, vinylether, vinyl ester, dicyclopentadienyl, norbornenyl, isoprenyl,isopropenyl, allyl or butenyl groups; dicyclopentadienyl ether,norbornenyl ether, isoprenyl ether, isopropenyl ether, allyl ether orbutenyl ether groups; or dicyclopentadienyl ester, norbornenyl ester,isoprenyl ester, isopropenyl ester, allyl ester or butenyl ester groups,but especially acrylate groups, into the binders described above, suchincorporation taking place by means, for example, of polymer-analogousreactions.

An alternative option is to add constituents as commonly employed incoating materials curable with actinic radiation.

Examples of such constituents include

-   the binders envisaged for use in UV-curable clearcoat materials and    powder clearcoat materials and described in the European patent    applications EP 0 928 800 A1, EP 0 636 669 A1, EP 0 410 242 A1, EP 0    783 534 A1, EP 0 650 978 A1, EP 0 650 979 A1, EP 0 650 985 A1, EP 0    540 884 A1, EP 0 568 967 A1, EP 0 054 505 A1 or EP 0 002 866 A1, in    the German patent applications DE 197 09 467 A1, DE 42 03 278 A1, DE    33 16 593 A1, DE 38 36 370 A1, DE 24 36 186 A1 or DE 20 03 579 B1,    in the international patent applications WO 97/46549 or WO 99/14254    or in the American patents U.S. Pat. No. 5,824,373 A, U.S. Pat. No.    4,675,234 A, U.S. Pat. No. 4,634,602 A, U.S. Pat. No. 4,424,252 A,    U.S. Pat. No. 4,208,313 A, U.S. Pat. No. 4,163 810 A,

U.S. Pat. No. 4,129,488 A, U.S. Pat. No. 4,064,161 A or U.S. Pat. No.3,974,303 A;

-   reactive diluents curable with actinic radiation, such as those    described in Römpp Lexikon Lacke und Druckfarben, Georg Thieme    Verlag, Stuttgart, N.Y., 1998, on page 491 under the entry “Reactive    diluents”; or-   photoinitiators as described in Römpp Chemie Lexikon, 9th, expanded    and revised edition, Georg Thieme Verlag Stuttgart, Vol. 4, 1991, or    in Römpp Lexikon Lacke und Druckfarben, Georg Thieme Verlag,    Stuttgart, 1998, pages 444 to 446.

The slurry of the invention may be prepared by the processes asdescribed in the patent applications DE195 40 977 A1, DE 195 18 392 A1,DE 196 17 086 A1, DE-A-196 13 547, DE 196 18 657 A1, DE 196 52 813 A1,DE 196 17 086 A1, DE-A-198 14 471 A1, DE 198 41 408 A1, or DE 198 41 842A1 or C2. It is of advantage in accordance with the invention to preparethe slurry of the invention by means of the process described in DE 19841 842 A1 or C2.

In this process, the ionically stabilizable binders and the crosslinkingagents and also, if appropriate, the additives are mixed in organicsolution and dispersed together in water with the aid of neutralizingagents by the secondary dispersion process. The system is then dilutedwith water, while stirring. A water-in-oil emulsion is formed first ofall, which on further dilution changes to become an oil-in-wateremulsion. This point is generally reached at solids contents of <50% byweight, based on the emulsion, and is evident externally from arelatively sharp drop in viscosity in the course of dilution.

The emulsion thus obtained, which still contains solvent, issubsequently freed from solvents by means of azeotropic distillation.

The distillation temperature is guided primarily by the glass transitiontemperature Tg of the binder. In order to avoid coagulum, i.e.,coalescence of the particles, which are only slightly stabilized inaccordance with the invention, to form a separate continuous organicphase during the distillation, it is essential that the distillationtemperature be held below the glass transition temperature Tg. The glasstransition temperature may also be described, as a substitute, by theminimum film-forming temperature of the dispersion. The minimumfilm-forming temperature may be determined by drawing down thedispersion onto a glass plate using a bar coater and heating it in agradient oven. The temperature at which the pulverulent layer films isdesignated the minimum film-forming temperature.

In accordance with the invention it is of advantage if the minimumfilm-forming temperature is more than 20° C., in particular more than30° C. For further details, reference is made to Römpp Lexikon Lacke undDruckfarben, Georg Thieme Verlag, Stuttgart, N.Y., 1998, “Minimumfilm-forming temperature”, page 391.

It is of advantage in accordance with the invention if the solvents tobe removed are distilled off at a distillation temperature below 70° C.,preferably below 50° C. and in particular below 40° C. If appropriate,the distillation pressure is chosen so that in the case ofhigher-boiling solvents this temperature range is still maintained.

At its simplest, the azeotropic distillation may be realized by stirringthe emulsion at room temperature in an open vessel for several days. Inthe preferred case, the solvent-containing emulsion is freed from thesolvents by a vacuum distillation.

In order to avoid high viscosities, the amount of water and solventsremoved by distillation or evaporation is replaced by water. The watermay be added before, during or after the evaporation or distillation, inportions.

After the solvents have been lost, the glass transition temperature Tgof the dispersed particles rises, and instead of the previoussolvent-containing emulsion (liquid-in-liquid dispersion) asolid-in-liquid dispersion, i.e., the slurry of the invention, isformed.

Preferably, the particles of the resultant slurry are mechanicallycomminuted in the wet state, which is known as wet milling. In this caseit is preferred to employ conditions such that the temperature of themilled material does not exceed 70° C., preferably 60° C., and inparticular 50° C. Preferably, the specific energy input during themilling process is from 10 to 1 000, more preferably from 15 to 750, andin particular from 20 to 500 Wh/g.

For wet milling it is possible to employ a very wide variety ofequipment which produces high or low shear fields.

Examples of suitable equipment which produces low shear fields arecustomary and known stirred vessels, slot homogenizers, microfluidizers,and dissolvers.

Examples of suitable equipment which produces high shear fields arecustomary and known stirred mills and in-line dissolvers.

Particular preference is given to employing the equipment which produceshigh shear fields. Of these, the stirred mills are particularlyadvantageous in accordance with the invention and are therefore usedwith very particular preference.

During wet milling, generally, the slurry of the invention is suppliedto the above-described equipment and circulated therein by means ofappropriate devices, such as pumps, until the desired particle size hasbeen reached.

The slurry of the invention advantageously has a solids content of from10 to 60% by weight, in particular from 20 to 50% by weight.

The slurry of the invention is preferably filtered prior to its use.This is done using the customary and known filtration equipment andfilters, as also suitable for filtering the known powder clearcoatslurries. The mesh size of the filters may vary widely and is guidedprimarily by the particle size and particle-size distribution of theparticles of the slurry of the invention. The skilled worker willtherefore easily be able to determine the appropriate filters on thebasis of this physical parameter. Examples of suitable filters are bagfilters. These are available on the market under the brand names Pong®or Cuno®. It is preferred to use bag filters having mesh sizes of from10 to 50 μm, examples being Pong® 10 to Pong® 50.

To produce the clearcoats of the invention, the slurry of the inventionis applied to the substrate that is to be coated. No special measuresneed be taken here; instead, the application may take place inaccordance with the customary and known techniques, which is anotherparticular advantage of the slurry of the invention.

Following its application, the slurry of the invention dries withoutproblems and does not film at the processing temperature, generally atroom temperature. In other words, the slurry of the invention applied asa wet film loses water when flashed off at room temperature or slightlyelevated temperatures, without the particles present therein alteringtheir original solid form. The solid film in powder form loses theresidual water by evaporation more easily than a flowing wet film. As aresult, the risk of bubbles of evaporated water enclosed in the curedfilm (popping) is reduced. Moreover, the tendency toward mudcracking isextremely low. A surprising finding in this context is that themudcracking tendency of the slurries of the invention is lower thehigher their particle sizes.

In the subsequent baking step, the now substantially water-free powderlayer is melted and caused to crosslink. In some cases, it may be ofadvantage to carry out the leveling process and the crosslinkingreaction with a chronological offset, by operating in accordance with astaged heating program or a so-called heating ramp. The appropriatecrosslinking temperature for the present examples is between 120 and160° C. The corresponding baking time is between 20 and 60 minutes.

In the case of the dual-cure slurry of the invention, thermal curing issupplemented by curing with actinic radiation, using the customary andknown radiation sources.

The clearcoat which results in this case has outstanding performanceproperties. For instance, it adheres firmly to all customary and knownbasecoats or to substrates such as metal, glass, wood, ceramic, stone,concrete or plastic. It is of high gloss, smooth, scratch-resistant,stable to weathering and chemicals, and even at high coat thicknesses isfree from defects such as stress cracks or popping marks. It no longerexhibits any blushing on exposure to moisture.

On the basis of this advantageous profile of properties, the slurry ofthe invention is outstandingly suitable for automotive OEM finishing,automotive refinishing, the interior and exterior coating ofconstructions, the coating of doors, windows and furniture, andindustrial coating, including coil coating, container coating and theimpregnation and/or coating of electrical components. It is used inparticular to produce clearcoats as part of multicoat color and/oreffect coating systems, which are produced from basecoat materials andthe slurry of the invention in accordance with the customary and knownwet-on-wet techniques.

EXAMPLES AND COMPARATIVE EXPERIMENTS Preparation Example 1 (Comparative)

The Preparation of the Solution Polyacrylate Resin A in Accordance withPreparation Example 1, Section 1.1, of DE 198 41 842 C2

1 291.5 parts by weight of methyl isobutyl ketone (MIBK) and 43.0 partsby weight of mercaptoethanol were introduced into a reaction vessel andheated to 100° C. The initiator, consisting of 143.5 parts by weight ofTBPEH (tert-butyl perethylhexanoate) and 86.1 parts by weight of MIBK,and the monomer mixture, consisting of 485.0 parts by weight oftert-butyl acrylate, 254.0 parts by weight of n-butyl methacrylate,213.8 parts by weight of cyclohexyl methacrylate, 409.0 parts by weightof hydroxypropyl methacrylate and 73.2 parts by weight of acrylic acid,were metered into this initial charge at 100° C. over the course of 5 hfrom two separate feed vessels. The reaction mixture was then heated to110° C., and a fraction of the volatile components of the reactionmixture was stripped off under reduced pressure at 500 mbar for 5 h. Theresin solution was then cooled to 80° C. and discharged.

The resin solution had the following characteristics:

Solids: 70.2% (1 h at 130° C.) Viscosity: 25.5 dPas (cone and plateviscometer at 23° C.; 70% strength solution) Acid number: 43.4 mg KOH/gresin solids OH number: 110 mg KOH/g resin solids.

Preparation Example 2 (Comparative)

The Preparation of the Solution Polyacrylate Resin B in Accordance withPreparation Example 1, Section 1.2, of DE 198 41 842 C2

1 076.7 parts by weight of methyl isobutyl ketone (MIBK) and 35.9 partsby weight of mercaptoethanol were introduced into a reaction vessel andheated to 100° C. The initiator, consisting of 119.6 parts by weight ofTBPEH (tert-butyl perethylhexanoate) and 71.8 parts by weight of MIBK,and the monomer mixture, consisting of 404.2 parts by weight oftert-butyl acrylate, 211.7 parts by weight of n-butyl methacrylate,239.2 parts by weight of cyclohexyl methacrylate and 340.9 parts byweight of hydroxypropyl methacrylate, were metered into this initialcharge at 100° C. over the course of 5 h from two separate feed vessels.The reaction mixture was then heated to 115° C., and a fraction of thevolatile components of the reaction mixture was stripped off underreduced pressure at 500 mbar for 3 h. The resin solution was then cooledto 80° C. and discharged.

The resin solution had the following characteristics:

Solids: 71.3% (1 h at 130° C.) Viscosity: 19.2 dPas (cone and plateviscometer at 23° C.; 70% strength solution) Acid number: 5 mg KOH/gresin solids OH number: 110 mg KOH/g resin solids.

Preparation Example 3

The Preparation of a Solution Polyacrylate Resin C Which can be Used inAccordance with the Invention

412 parts by weight of methyl ethyl ketone were introduced into areaction vessel and heated to 80° C. The initiator, consisting of 49parts by weight of VAZO® 67 (azobisisovaleronitrile) and 49 parts byweight of methyl ethyl ketone, and the monomer mixture, consisting of137.2 parts by weight of tert-butyl acrylate, 73.5 parts by weight ofn-butyl methacrylate, 98 parts by weight of cyclohexyl methacrylate,171.5 parts by weight of hydroxyethyl methacrylate and 9.8 parts byweight of acrylic acid, were metered into this initial charge at auniform rate at 80° C. with stirring over the course of six hours fromtwo separate feed vessels. The reaction mixture was then held at 80° C.for 1.5 hours. A fraction of the volatile components of the reactionmixture was then stripped off under reduced pressure for five hoursuntil the solids content was 70% by weight. The resultant resin solutionwas cooled to 50° C. and discharged.

The resin solution had the following characteristics:

Solids: 69.2% (1 h at 130° C.) Viscosity: 3.8 dPas (cone and plateviscometer at 23° C.; 55% strength solution, diluted with xylene) Acidnumber: 9.8 mg KOH/g resin solids OH number: 150 mg KOH/g resin solids.

Preparation Example 4

The preparation of a Blocked Polyisocyanate Based on IsophoroneDiisocyanate

837 parts by weight of isophorone diisocyanate were introduced into anappropriate reaction vessel, and 0.1 part by weight of dibutyl tindilaurate was added. A solution of 168 parts by weight oftrimethylolpropane and 431 parts by weight of methyl ethyl ketone wasthen run in slowly. As a result of the exothermic reaction, thetemperature rose. After it had reached 80° C., the temperature was keptconstant by external cooling and the rate of addition of the feed streamwas reduced slightly if necessary. After the end of the feed stream, themixture was held at this temperature for about 1 hour until theisocyanate content of the solids had reached 15.7% (based on NCOgroups). The reaction mixture was subsequently cooled to 40° C. and asolution of 362 parts by weight of 3,5-dimethylpyrazole in 155 parts byweight of methyl ethyl ketone was added over the course of 30 minutes.After the reaction mixture had heated up to 80° C., owing to theexothermic reaction, the temperature was kept constant for 30 minutesuntil the NCO content had dropped to less than 0.1%. Then 47 parts byweight of n-butanol were added to the reaction mixture, which was heldat 80° C. for a further 30 minutes and then, after brief cooling, wasdischarged.

The reaction product had a solids content of 69.3% (1 h at 130° C.).

Preparation Example 5

The Preparation of a Blocked Polyisocyanate Based on HexmethyleneDiisocyanate

534 parts by weight of Desmodur® N 3300 (commercial isocyanurate ofhexamethylene diisocyanate from Bayer AG) and 200 parts by weight ofmethyl ethyl ketone were introduced into a reaction vessel and heated to40° C. 100 parts by weight of 2,5-dimethylpyrazole were added to thissolution, with cooling, and the exothermic reaction was allowed tosubside. Subsequently, with continued cooling, a further 100 parts byweight of 2,5-dimethylpyrazole were added. After the exothermic reactionhad again subsided, a further 66 parts by weight of 2,5-dimethylpyrazolewere added. Thereafter, cooling was shut off, as a result of which thereaction mixture heated up slowly to 80° C. It was held at thistemperature until its isocyanate content had dropped to below 0.1%. Thereaction mixture was subsequently cooled and discharged.

The resultant solution of the blocked polyisocyanate had a solidscontent of 81% by weight (1 h at 130° C.) and a viscosity of 3.4 dPas(70% strength in methyl ethyl ketone; cone and plate viscometer at 23°C.).

Comparative Experiments C1 and C2

The Preparation of the Noninventive Powder Clearcoat Slurries C1 and C2

Comparative Experiment C1

The Preparation of the Noninventive Powder Clearcoat Slurry C1 on theBasis of the Solution Polyacrylate Resin A from Preparation Example 1

812.1 parts by weight of the acrylate resin solution A from PreparationExample 1 and 492.5 parts by weight of the solution of the blockpolyisocyanate from Preparation Example 4 were mixed at room temperaturein an open vessel for 15 minutes with stirring. Then 16.2 parts byweight of Cyagard 1164 (UV absorber from Cytec), 9.6 parts by weight ofTinuvin liquid 123 (sterically hindered amine “HALS” from Ciba Geigy),15.2 parts by weight of N,N-dimethylethanolamine and 7.0 parts by weightof dibutyltin dilaurate (DBTL) were added and the mixture was stirred atroom temperature for a further 2 h. The mixture was then diluted with561.3 parts by weight of deionized water in small portions. After aninterval of 15 minutes, a further 676.0 parts by weight of DI water wereadded. This gave an aqueous emulsion of low viscosity with a theoreticalsolids content of 37% which was stirred at room temperature for afurther 48 hours. The amount of liquid evaporated off was supplementedby adding DI water until the original level was regained. This gave apowder clearcoat slurry having the following characteristics:

Solids (2 h, 80° C.): 35.6% Carboxyl group content: 0.52 meq/g solidsNeutralizing agent content: 0.22 meq/g solids Degree of neutralization:42% Solvent content: <0.05% (by gas chromatography) Particle size: 6 μm(D.50; laser diffractometer from Malvern)

In order to produce the desired pseudoplasticity, 8.7 parts by weight ofAcrysol® RM 8 (nonionic associative thickener from Rohm & Haas) and 6.0parts by weight of Viskalex® (thickener from Allied Colloids) wereincorporated by stirring into 1 000 parts by weight of the powderclearcoat slurry.

The viscosity profile of the resultant powder clearcoat slurry 1 was asfollows:

-   1 405 mPas at a shear rate of 10 s⁻¹-   791 mPas at a shear rate of 100 s⁻¹-   308 mPas at a shear rate of 1 000 s⁻¹

The powder clearcoat slurry C1 had a minimum film-forming temperature of35° C.

Comparative Experiment C2

The Preparation of the Noninventive Powder Clearcoat Slurry C2 on theBasis of the Solution Polyacrylate Resins A and B from PreparationExamples 1 and 2

331.0 parts by weight of the acrylate resin A from Preparation Example1, 774.5 parts by weight of the acrylate resin B from PreparationExample 2 and 715.8 parts by weight of the solution of the blockedpolyisocyanate from Preparation Example 4 were mixed with one another asdescribed in Example 1. Then 4.8 parts by weight of Cyagard 1146, 7.6parts by weight of Tinuvin 123, 10.0 parts by weight ofN,N-dimethylethanolamine and 5.5 parts by weight of DBTL were added.After 2 hours of stirring, 723.0 parts by weight of DI water were addedin small portions and the resultant mixture was diluted 15 minutes laterwith a further 910.0 parts by weight of DI water. The resulting powderclearcoat dispersion was transferred to a reactor and the solvent wasremoved as an azeotrope with the accompanying water under reducedpressure at from 25 to 35° C., the amount of distillate being replacedin the course of the distillation by 2 000 parts by weight of DI waterin small portions, by way of a vacuum dropping funnel with a three-waytap. The distillation was continued until residual solvent was no longerdetectable. The characteristics of the resulting powder clearcoat slurrywere as follows:

Solids (2 h, 80° C.): 44.3% Carboxyl group content: 0.19 meq/g solidsNeutralizing agent content: 0.10 meq/g solids Degree of neutralization:52% Solvent content: <0.05% (by gas chromatography) Particle size: 7 μm(D.50; laser diffractometer from Malvern)

In order to produce the desired pseudoplasticity, 7.8 parts by weight ofAcrysol® RM 8 and 4.7 parts by weight of Viskalex HV 30 wereincorporated by stirring into 1 000 parts by weight of the powderclearcoat slurry. The viscosity profile of the resultant powderclearcoat slurry 2 was as follows:

-   5 243 mPas at a shear range of 10 s⁻¹-   569 mPas at a shear range of 1 000 s⁻¹

The minimum film-forming temperature was 43° C.

Comparative Experiments C3 and C4

The use of the Noninventive Powder Clearcoat Slurries C1 (ComparativeExperiment C1) and C2 (Comparative Experiment C2) to Produce theNoninventive Clearcoats C3 and C4

The powder clearcoat slurries C1 and C2 were applied using a so-calledintegrated system, which is described below for the metallic shade“meteor gray”:

Using a gravity feed gun, a functional coat (Ecoprime® Meteorgrau[meteor gray]; BASF Coatings AG) was applied to steel panels coatedcathodically with a commercial electrocoat material. After flashing offat room temperature for 5 minutes, a meteor gray aqueous metallicbasecoat (Ecostar® Meteorgrau; BASF Coatings AG) was applied in the sameway to this coat and was subsequently predried at 80° C. for 5 minutes.

After the panels had been cooled, the noninventive powder clearcoatslurries C1 (Comparative Experiment C3) and C2 (Comparative ExperimentC4) were applied in the same way. Thereafter, the panels were firstflashed off for 5 minutes and then predried at 40° C. for 15 minutes.They were subsequently baked at 145° C. for 30 minutes.

In the case of Comparative Experiment C3, this gave an aqueous metallicoverall coating system in the shade “meteor gray”. The applied wet filmshad been chosen so that, after baking, the dry film thicknesses for thefunctional coat and for the aqueous metallic basecoat were each 15 μm.The clearcoat C3 had a film thickness of from 40 to 45 μm.

The second panel, prepared in the same way, with the powder clearcoatslurry C2 (Comparative Experiment C4) again had a film thickness of 15μm in each case for the functional coat and for the aqueous metallicbasecoat. The clearcoat 3 had a film thickness of from 44 to 48 μm.

The performance properties of the noninventive clearcoats C3 and C4 arecompared in the table with the performance properties of thenoninventive clearcoat C6 and the inventive clearcoat.

Comparative Experiment C5

The Preparation of the Noninventive Powder Clearcoat Slurry C5

855.2 parts by weight of the acrylate resin solution from PreparationExample 3, 205.1 parts by weight of the solution of the blockedpolyisocyanate from Preparation Example 4 and 415.1 parts by weight ofthe solution of the blocked polyisocyanate from Preparation Example 5were mixed at room temperature in an open vessel for 15 minutes withstirring. Then 42.9 parts by weight of Cyagard® 1664L (commercial UVabsorber from Cytec), 10.7 parts by weight of Tinuvin® 123 (stericallyhindered amine “HALS” from Ciba), 5.3 parts by weight of benzoin, 7.835parts by weight of N,N-dimethylethanolamine and 0.857 part by weight ofdibutyl tin dilaurate were added to the resultant mixture and wasstirred at room temperature for a further two hours. The mixture wasthen diluted with 676.9 parts by weight of deionized water in smallportions. After an interval of 15 minutes, a further 780 parts by weightof DI water were added at a uniform rate over the course of 30 minutes.This gave an aqueous emulsion of low viscosity with a theoretical solidscontent of 37% by weight.

The aqueous emulsion was then diluted with a further 425.2 parts byweight of deionized water. Thereafter, under reduced pressure on arotary evaporator, the same amount of a mixture of volatile organicsolvents in water was removed from it, after which the solids contentwas again at 37% by weight (1 h at 130° C).

The characteristics of the resulting powder clearcoat slurry were asfollows:

Solids (1 h at 130° C.): 36.9% Carboxyl group content: 0.096 meq/gsolids Neutralizing agent content: 0.079 meq/g solids Degree ofneutralization: 85% Solvent content: <0.05% (by gas chromatography)Particle size: 0.9 μm (D.50; laser diffractometer from Malvern)

In order to produce the desired pseudoplasticity, 79 parts by weight ofAcrysol® RM 8 and 22.7 parts by weight of Viskalex HV 30 wereincorporated by stirring into the powder clearcoat slurry.

Comparative Experiment C6

The Production of a Noninventive Clearcoat C6

Comparative Experiment C3 was repeated but using the powder clearcoatslurry C5 from Comparative Experiment C5 instead of the powder clearcoatslurry C1, with a film thickness of the clearcoat of from 39 to 44 μm.

The performance properties of the noninventive clearcoat C6 are comparedin the table with the performance properties of the noninventiveclearcoats C3 and C4 and the inventive clearcoat from Example 2.

Example 1

The Preparation of an Inventive Powder Clearcoat Slurry

Comparative Experiment C5 was repeated but using 4.148 parts by weightof N,N-dimethylethanolamine rather than 7.835 parts by weight, and withthe film thickness of clearcoat being from 45 to 49 μm.

Prior to the adjustment of the viscosity, the characteristics of theinventive powder clearcoat slurry were as follows:

Solids (1 h at 130° C.): 36.7% Carboxyl group content: 0.096 meq/gsolids Neutralizing agent content: 0.045 meq/g solids Degree ofneutralization: 45% Solvent content: <0.05% (by gas chromatography)Particle size: 4.8 μm (D.50; laser diffractometer from Malvern)

Example 2

The Production of an Inventive Clearcoat

Comparative Experiment C6 was repeated but using the inventive powderclearcoat slurry from Example 1 rather than the noninventive powderclearcoat slurry C5.

The performance properties of the inventive clearcoat from Example 2 arecompared in the table with the performance properties of thenoninventive clearcoats C3, C4 and C6.

TABLE The performance properties of the inventive clearcoat (Example 2)and of the noninventive clearcoats C3, C4 and C6 Comparativeexperiments: Property C3 C4 C6 Example 2 Film thickness (μm) 40–45 40–4839–44 45–49 Gloss at 20°*⁾ 77 77 81 84 Haze**⁾ 80 84 41 24 Appearancebright glossy blisters bright without cracks defect Leveling very goodgood good very good Chemical resistance:**⁾ 1% strength sulfuric acid 5646 40 42 Pancreatin 54 >58   42 43 Tree resin 44 41 43 47 Water 54 4955 >62   Popping limit (μm)***⁾ 45 48 45 65 *⁾Measuring instrument,manufacturer Byk; **⁾Measurement by means of gradient oven, manufacturerByk. The numerical value indicates the lower temperature in ° C. abovewhich drops of the corresponding substance applied to the coating leftvisible traces; ***⁾Determined by wedge application.

A comparison of the results in the table shows that the inventiveclearcoat has a significantly higher resistance to moisture and asignificantly higher popping limit, in combination with better gloss andhaze. Whereas blushing was observed in the case of the noninventiveclearcoats C3, C4 and C6, it no longer occurred with the inventiveclearcoat.

1. A pseudoplastic powder clearcoat slurry free from organic solventsand external emulsifiers, comprising particles which are dimensionallystable under storage and application conditions, the particles having anaverage size of from 1.0 to 20 μm and a particle size distributionwherein at least 99% of the particles have a size <30 μm, and a binder,said binder comprising at least one polyol comprising ion-forming groupsconvertible into salt groups by neutralization and an OH number of from130 mg KOH/g to 160 mg KOH/g, wherein said pseudoplastic powderclearcoat slurry has a viscosity of (i) from 50 to 1 000 mPas at a shearrate of 1 000 s⁻¹, (ii) from 150 to 8 000 mPas at a shear rate of 10s⁻¹, and (iii) from 180 to 12 000 mPas at a shear rate of 1 s⁻¹ andcomprises from 0.01 to 1 meq/g of solids of ion-forming groupsconvertible into salt groups by neutralization, and from 0.005 to 0.1meq/g of solids of ionic groups produced by neutralization of theion-forming group at not more than 50% neutralization.
 2. The slurry ofclaim 1, wherein the polyol comprises a (meth)acrylate copolymer.
 3. Theslurry of claim 1, comprising a solids content of from 10 to 60% byweight.
 4. The slurry of claim 1, further comprising ionic thickenersand nonionic associative thickeners.
 5. The slurry of claim 1, whereinthe particles comprise a crosslinking agent selected from the groupconsisting of blocked polyisocyanates,tris(alkoxycarbonylamino)triazines and mixtures thereof.
 6. The slurryof claim 1, having a minimum film-forming temperature of more than 20°C.
 7. A method of making a coated substrate, comprising applying theslurry of claim 1 to a substrate selected from the group consisting ofautomotive OEM finishing substrates, automotive refinishing substrates,interior construction substrates, exterior construction substrates, doorsubstrates, window substrates, furniture substrates, industrial coatingsubstrates, coil coating substrates, container coating substrates, andelectrical component substrates.